avcodec/ituh263dec: make while get_bits loop more robust by checking bits left

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ituh263dec.c

@@ -1115,6 +1115,8 @@ int ff_h263_decode_picture_header(MpegEncContext *s)
     /* PEI */
     while (get_bits1(&s->gb) != 0) {
         skip_bits(&s->gb, 8);
+        if (get_bits_left(&s->gb) <= 0)
+            return AVERROR_INVALIDDATA;
     }
 
     if(s->h263_slice_structured){